1. Technical Field of the Invention
This invention relates to well logging tools for emitting a series of neutron bursts into a formation and detecting gamma radiation emitted by the formation in response to the neutron bursts, and in particular, to a well logging tool for measuring inelastic energy spectra of the resultant gamma radiation.
2. Description of the Prior Art
Prior art well logging tools have been used for emitting pulses of fast neutrons and detecting a resultant gamma radiation from a formation. Fast neutrons are generated in pulses, or bursts, of neutron emissions and typically have an initial energy level of 14 MeV. The detected resultant gamma radiation is utilized to determine various formation parameters, such as porosity, lithology, water saturation, and hydrocarbon saturation.
These prior art well logging tools are typically operated in either a sigma logging mode or a spectroscopy logging mode. Prior art well logging tools which are operated in the sigma logging mode have been used during the last thirty years for logging wells to determine thermal neutron capture cross sections, or sigma, for formations. During the last twenty years, prior art well logging tools which are operated in the spectroscopy logging mode have been developed for measuring inelastic energy spectra for the gamma rays which are emitted as a result of inelastic collisions between the fast neutrons and formation nuclei.
The inelastic energy spectra measured during a spectroscopy logging mode contain signature energy peaks which may be processed to determine the relative amounts of particular types of nuclei included within a formation. For example, when nuclei of carbon and oxygen are impacted by fast neutrons having energy levels which range from 5 MeV to 14 MeV, or greater, gamma radiation having gamma rays at discrete energy levels are emitted as a result of the impacts, or collisions. These discrete energy levels of emitted gamma rays are different for nuclei of carbon than for nuclei of oxygen, providing signature peaks in the detected energy spectra for a formation interval.
Prior art logging tool detectors have been used for measuring inelastic spectra to determine the relative amounts of these discrete energy levels from gamma radiation energy spectrum data samples. Although these discrete energy levels are "smeared" together by compton scattering and the response characteristics of logging tool detector means, they still may be used to provide characteristics of the formation, such as, for example, a Carbon/Oxygen ratio, or "C/O" ratio. Other ratios may also be determined. These ratios may then be used for determining formation parameters, such as distinguishing tight formations from gas, oil from fresh water, and determining lithology.
Prior art well logging tools typically determine an inelastic energy spectrum by taking a total gamma ray energy spectrum data sample during a series of neutron bursts, and then deducting a thermal neutron capture energy spectrum from the total gamma ray energy spectrum. The thermal neutron capture energy spectrum is determined by detecting thermal neutron capture energy spectral counts during the decay periods following the neutron bursts, and then, in some instances, multiplying these spectral counts by a fixed scaling factor. This method for determining the thermal neutron capture energy spectrum is less accurate than if a gamma ray energy decay rate were to be determined for the decay periods of the data sample and used for determining the capture component of the gamma ray energy spectrum.
These prior art well logging tools, when operable in both spectroscopy and sigma logging modes, are operated in only one mode during a single logging pass, since different tool firing cycles are required to obtain an optimum statistical quality for data samples. This requires that at least two logging passes be run in order to obtain measurements for both neutron capture cross sections and inelastic energy spectra having a desired statistical quality.
Further, spectroscopy logging modes for tool operating cycles of prior art well logging tools include firing cycles which have decay periods which do not extend for several lifetimes of thermal neutrons within a formation. This results in data samples which are less accurate for determining thermal neutron decay rates during prior art spectroscopy logging modes than if the decay periods were extended for several lifetimes, as some prior are well logging tools do when operated in a capture operating mode.
Additionally, tool operating cycles for operating prior art well logging tools in spectroscopy logging modes generally include firing cycles having short neutron burst durations, as compared to some prior art well logging tools when operated in sigma mode. A longer spectroscopy mode neutron burst duration allows for more accurate measurement of inelastic energy spectra.